1. Field of the Invention
This invention relates to the detonation of an explosive device and, more particularly, to the control of a plurality of detonators having varying detonation delays.
2. Description of Related Art
In the blasting of rock in mining, quarrying or construction operations, it is necessary to place discrete explosive charges within boreholes drilled within the mass of the rock, such that the detonation of each individual charge can act effectively on the rock to both fracture and move the rock, without producing levels of vibration in the surrounding rock sufficient to cause damage or nuisance to an adjacent property. It is, therefore, necessary to utilize an array of blasting caps or detonators, with one or more plates within each individual charge such that each charge fires in a pre-determined sequence and with such a time delay interval between other charges that they fire independently of each other.
At present, it is common to use blasting caps (detonators) with different delay periods produced by the burning of pyrotechnic delay elements of various lengths and with varying compositions such that the time between the blasting cap receiving a firing signal and the detonation of the base charge can be determined during manufacture within certain tolerances. Such initiation systems have several problems associated with them. Since different detonation delays are required, it is common to provide a large number of blasting caps (detonators) with different time delays. For example, thirty detonation delays, in 25 or 30 msec increments, are common in the industry. The desired time delay is determined for each borehole and the detonator (blasting cap) possessing the desired time delay is installed in the borehole along with the charge. Moreover, the lead wires that connect the detonator to the top of the borehole are typically hard-wired to the detonator and the length of the lead wires must vary for the various depths of the boreholes. Ten or fifteen separate lead wire lengths are usually manufactured to meet the need of differing depths of boreholes. As a result, an installer must have available a multiplicity of detonators, up to 400 different versions or units, possessing the various combinations of available time delays and various lead wire lengths, and install a particular detonator (time delay/lead wire length) in each borehole. The inventory required of the various time delays/lead wire lengths for the various detonators is quite large. Moreover, lack of the correct delay time or lead wire length can result in the use of an inappropriate detonator to initiate a particular charge or group of charges. The delay timings are set during manufacture, which limits the scope of obtaining the most efficient or appropriate timing of the charges within a particular blast. Indeed, due to the limitations inherent in the manufacture of such pyrotechnic delays, blasting caps or detonators of the same nominal delay time can vary quite considerably. The effects of temperature, humidity, age, storage, and handling all contribute to degradation in the accuracy of the delay time actually produced at the time of actual detonation. This can result in out of sequence firing of the individual explosive charges, which can produce fly-rock, poor fragmentation of rock, and/or high levels of ground vibration and air blast.
Electric blasting caps or detonators will initiate the detonation of an explosive charge if it is supplied with sufficient electrical energy from a source. Of necessity, the energy levels required are relatively low. Stray electrical energy from radio transmissions, static electrical build-up, earth leakage from faulty equipment and nearby lightning strikes have all been responsible for premature detonation of electric detonators. Non-electric systems have been used to overcome most of these problems, but they suffer from the drawback that it is impossible to test that the circuit is intact and correctly connected prior to attempting to fire the blast. Even with electric detonators it is impossible to check the functionality of the delay element. As a result, a small proportion of detonators will misfire, producing the hazardous situation where unexploded explosives remain hidden amongst the rock pile without anyone realizing that they are present.
Other means have been used to improve the safety and reliability of the electric delay detonator, including a transformer coupling which resulted in a much simplified method of connecting the detonators into the firing circuit while at the same time overcoming many of the problems due to stray electrical energy and current leakage. Devices known as the xe2x80x9cMagnadetxe2x80x9d detonator allowed for a significant reduction in inventory to be made by providing a system where lead wires could be coupled to a standard shot-length detonator unit in the field. See, for example, U.S. Pat. Nos. 4,297,947 and 4,425,849. However, the problems associated with delay time accuracy can only be addressed by moving away from traditional pyrotechnic delay systems.
Although not yet routinely applied in mining and quarrying operations, the use of electronically timed detonators does provide a solution to the problems of delay time accuracy and the ability for the blaster to determine the delay time of each unit. See, for example, U.S. Pat. Nos. 4,324,182; 4,409,897; 4,646,640; 5,189,246; 5,282,421; 5,406,890; 5,520,114; and 5,602,713. Although inventory levels are reduced due to the absence of pre-set or nominal delay periods, the requirement for manufactured lead wires of different length and/or special connectors creates new problems with stocking the correct components and having the skilled personnel available to correctly employ special connectors to provide a reliable electrically competent connection.
Other relevant patents include U.S. Pat. Nos. 5,460,093; 5,295,438; 5,214,236; 4,893,564; 4,860,653; 4,674,047; 4,601,243; 4,586,437; 4,311,096 and 4,145,970.
In summary, there is a need for improved timing accuracy of blasting caps or detonators together with a need for an ability to set the nominal delay time of each detonator appropriate to its location within the blast in order to obtain more controllable rock fragmentation and displacement and the reduction of undesirable ground vibrations. Also, in order to improve safety and reliability, it would be beneficial to minimize the susceptibility of electric blasting systems to extraneous electrical stimuli, while simplifying the connection of the devices into the blasting circuit, and being able to use standard, readily available cabling and connectors. Reliability could be further improved by being able to test the functionality of each blasting cap prior to it being incorporated into the blasting circuit. Ideally, a single programmable detonator or blasting cap with a simple, reliable means of connection into the blasting circuit would ensure that the most appropriately timed detonator will be correctly located within the blast, in order to provide the most efficient method of blasting. It would also be extremely cost-effective to reduce the detonator inventory to only one basic programmable detonator unit which can be connected into the blasting circuit, at any desired position, by reels of readily available standard insulated conductive wire.
Accordingly, we have developed an electronic delay assembly which can be connected to an explosive detonator and effect the firing of the detonator in a controlled manner. The electronic delay assembly in accordance with our invention includes a magnetic coupling device having an opening therein configured to receive a conductive wire extending therethrough. The magnetic coupling device generates output signals based on currents passing in the wire. The assembly also includes a system power reservoir connected to the magnetic coupling device and storing electrical energy therein based on power signals passing in the wire extending therethrough and generated by the magnetic coupling device. The assembly also includes a microprocessor which has internal common nonvolatile memory therein and which receives its operating power from the system power reservoir. The assembly also includes a decoder which is connected to the magnetic coupling device, which decodes communications signals passing in the wire extending therethrough and generated by the magnetic coupling device, and supplies those decoded communications signals to the microprocessor. In addition, the assembly includes a trigger circuit connected between the system power reservoir and a fuse head in the explosive detonator for supplying, under the control of the microprocessor, electrical energy from the system power reservoir sufficient to fire the detonator connected thereto.
In a preferred embodiment, the assembly further includes a wireless communications link connected to and controlled by the microprocessor. The communications link provides information regarding the current status of the operation of the microprocessor or data stored therein. For example, the wireless communications link can be an infrared light emitting diode. In addition, the communications signals passing through the wire and generated by the magnetic coupling device can include timing signals from an external programming device. The timing signals are stored in the nonvolatile memory of the microprocessor and form a detonation time delay for the electronic assembly. With the wireless communications link, the detonation time delay stored in the microprocessor can be supplied back to the programming device through the communications link to confirm the accuracy of the detonation time delay provided to the microprocessor.
The power signals generated by the magnetic coupling device can be supplied to a power rectifier which supplies its output power to the system power reservoir. In a preferred embodiment, the power rectifier is a full wave diode bridge rectifier. In addition, the system power reservoir can be a capacitor and the decoder can be a pulse discriminator. A preferred magnetic coupling device for the present invention is a toroidal transformer.
The assembly can further include a clock that supplies timing signals to the microprocessor and a power regulator that receives power from the system power reservoir and supplies regulated voltage to the microprocessor. A low voltage threshold can be provided to monitor the voltage on the system power reservoir and supply this voltage to the microprocessor, such that if the voltage on the system power reservoir drops below a predetermined value, the microprocessor will fire the trigger circuit and supply power to the fuse head, only after a valid fire message had been received.
The trigger circuit can include a pair of switches linked together in a way such that both switches must be activated by the microprocessor before power is supplied from the system power reservoir to the fuse head. In one embodiment, the trigger circuit can include four circuits that form a pair of switches, including a high side hard drive, a low side hard drive, a high side soft drive and a low side soft drive. In one embodiment of the assembly, the communications signals passing through the wire and generated by the magnetic coupling device include test signals for testing the functioning of the four drives in the trigger circuit. In this manner, if any drive has a fault therein, the assembly will not accidentally trigger the passage of power to the fuse head and cause an accidental explosion.
In a preferred embodiment, the communications signals passing through the wire and generated by the magnetic coupling device include timing signals which store in the nonvolatile memory of the microprocessor a specific detonation time delay and, thereafter, include control signals for activating the electronic assembly to fire, at the pre-programmed time delay, a detonator attached thereto.
We have also invented a method of programming a detonation time delay into the electronic delay assembly described above as well as a method of conducting a blasting operation using the electronic delay assembly discussed above and a detonator attached thereto.